I’m having the same issue with one of my websites (working perfectly fine on others), for no apparent reason. The sitemap causing trouble is: https://belgrademyway.com/post-sitemap.xml
It came to my mind that the image (1px) somehow got embedded in a post, but can’t find it with Google, nor in the sitemap itself (image paths not displayed in the sitemap).
Robots.txt itself isn’t the problem, since it’s identical to other WP based websites, hiding wp-admin and wp-includes folders from the bots.
Anyone knows how to solve this issue?